A FCBGA (flip-chip ball grid array) semiconductor package is formed with both a flip-chip structure and a ball grid array in which at least one chip is mounted and electrically connected to a surface of a substrate by a plurality of solder bumps, and a plurality of solder balls are implanted on an opposite surface of the substrate to serve as input/output (I/O) connections of the semiconductor package. In order to dissipate heat produced from operation of the chip, a heat sink is normally incorporated in the semiconductor package, as disclosed by U.S. Pat. Nos. 5,311,402, 5,637,920, 5,931,222 and 6,011,304. This heat sink is attached to the substrate by means of an adhesive or solder and is usually greater in surface area than the chip to cover the chip and effectively dissipate the heat from the chip. Further, as to enhance electrical performances of the semiconductor package, at least one passive component can be mounted on the substrate, which however would decrease area on the substrate capable of being in contact with the heat sink, thereby making the heat sink relatively difficult to be firmly adhered and positioned onto the substrate and thus leading to dislocation of the heat sink; this situation becomes severe in the use of a large heat sink. Moreover, attachment between the heat sink and the substrate via the adhesive or solder would be damaged by unsatisfactory cleanness of contact surfaces between the heat sink and the substrate, or by undesirable stress applied to the heat sink and the substrate; in this case, delamination may occur at an interface between the heat sink and the substrate, and as a result, the heat sink is dislocated. Furthermore, when the substrate mounted with the heat sink is subject to external force such as vibration or shock, the heat sink may also be dislocated from the substrate.
Therefore, it has been taught to mount a heat sink on a chip or to mechanically secure a heat sink on a substrate in position. The former is illustrated by U.S. Pat. No. 6,093,961; as shown in FIG. 5, a heat sink 22 is directly stacked on and coupled to a flip chip 21 on a substrate 20; the heat sink 22 is formed with a plurality of flexible legs 23 extending toward the chip 21, each leg 23 having a hook end 230. When the heat sink 22 is pressed on an upper surface 210 of the chip 21, the hook ends 230 of the flexible legs 23 are adapted to be engaged with corners of a lower surface 211 of the chip 21 to securely dispose the heat sink 22 onto the chip 21. However, this structure is defective that the chip may be damaged if improperly pressing the heat sink onto the chip; further, during a high temperature process or thermal cycle, due to mismatch in coefficient of thermal expansion (CTE) between the heat sink and the chip, the chip may suffer thermal stress and crack.
U.S. Pat. Nos. 5,396,403 and 5,926,371 propose to position the heat sink on the substrate in a mechanical manner that the heat sink is formed with a plurality of holes at positions supposed to be in contact with the substrate that is also formed with a plurality of corresponding holes, and a plurality of fixing members such as bolts are coupled to the holes to connect the heat sink and the substrate. However, this mechanical method of attaching the heat sink onto the substrate renders significant problems. One is that predetermined area on the substrate is required for forming the holes, which may affect trace routability on the substrate and make the substrate not able to be mounted with a full array of solder balls. Besides, formation of the holes would undesirably increase fabrication costs and process complexity of the substrate. Further, external moisture or contaminant may enter the holes of the substrate and thus degrade reliability of fabricated products.
Therefore, the problem to be solved herein is to securely position a heat sink on a substrate without leading to dislocation of the heat sink, damaging a chip mounted on the substrate or affecting trace routability on the substrate.